1. Field of the Invention
The present invention relates to polyester matrix powders containing carbon nanotube powders dispersed therein; to conductive masterbatches with homogeneous and smooth surfaces and the preparing process thereof; to conductive monofilaments formed from the said conductive masterbatches and the preparing process thereof; and to textiles prepared from the said conductive monofilaments.
2. Description of Related Arts
Carbon nanotubes have been known as graphitizing carbon tubes and are different from conventional carbons in that the carbon nanotubes have a specific character of L/D ratio and can be used as the best conductive materials. Generally, the larger the L/D ratio of carbon nanotubes is, the better the conductivity of the same is. However, if the L/D ratio is too large, problems occur during spinning process. For instance, if the arrangement or orientation of the carbon nanotubes is not good, the drawing process becomes difficult. Further, if the carbon nanotubes fail to pass the fiber filter test, the filaments formed therefrom easily break during spinning process.
In order to enhance the conductivity of polymer matrixes, carbon nanotubes having good dispersibility may be added. The carbon nanotubes are obtainable by a high-speed mechanic force, followed by uniformly dispersing the carbon nanotubes in the polymer matrixes. However, achieving the dispersing effect and enhancing the conductivity of the polymer matrixes via such a high-speed mechanic process are only effective when the concentration of carbon nanotubes is low. If the concentration of the carbon nanotubes is high, the uniform dispersing effect cannot be reached even though the conductivity is increased. On the other side, the longer the carbon nanotubes is, the better the increasing effect of their conductivity is. Nevertheless, if the long length of the carbon nanotubes is disadvantageous to the processability. Such a result is caused by the tanglement of the carbon nanotubes themselves.
Recently, the high-speed shear mixing and processing technology makes the addition of up to 15 wt. % of the carbon nanotubes in the polymer matrix possible. However, the carbon nanotubes have poor dispersibility in the polymer materials by their process.
In other conventional methods, carbon nanotubes are dispersed in a strong acid solution to shatter the aggregates of the carbon nanotubes via ultrasonic wave. Specifically, the aggregates may be shattered in a mixed acid solution containing H2SO4 and HNO3 in a ratio of 3:1 at 50° C. via ultrasonic wave over a time period of 24 hours. The carbon nanotubes treated by a strong acid solution are easy to produce a COOH group that may increase the dispersibility of carbon nanotubes. However, this method has disadvantages in that the carbon nanotubes treated by a strong acid solution cause defects on the surfaces of their structures, and thus the properties and functions of the carbon nanotubes will be greatly reduced.
The prior art, for instance, CN1475437A discloses a process for the preparation of a carbon nanotube paper, comprising the steps of: purifying carbon nanotubes, dispersing the carbon nanotubes and forming a carbon nanotube paper; wherein the carbon nanotubes are repeatedly treated until the impurities are removed and the carbon nanotubes are sufficiently dispersed. Such processes have several disadvantages, such as: 1) the procedure is very complicated and costly; 2) the surfaces of carbon nanotubes treated with a strong acid will be destroyed and the properties, such as antistatic ability, conductivity or strength, of the carbon nanotubes become poor; 3) it is difficult to disperse the carbon nanotubes in a solvent and the solvent used causes an environmental problem; and 4) the surfaces of the carbon nanotubes are destroyed due to the treatment with a strong acid and the yield is only about 30 to 60%, whereby the production cost of the carbon nanotubes is significantly increased.
CN1563526A discloses conductive fibers containing carbon nanotubes and the preparing process thereof. The conductive fibers comprise 80 to 99.9 wt. % of a polyester, 0.05 to 10 wt. % of carbon nanotubes and 0.05 to 10 wt. % of a coupling agent, wherein the coupling agent is selected from OP wax, montan wax, polyethylene vinyl acetate or aluminate. In this known technology, the carbon nanotubes are untangled under a strong shear force, thereby being homogeneously dispersed within the polyester matrix. In this process, only a lower content of carbon nanotubes is required for preparing conductive fibers. According to this process, the coupling agent is added after the polyester and carbon nanotubes are dried under vacuum, followed by mixing them at a high speed and at a temperature of 70 to 120° C. After that, masterbatches are prepared at a speed of 40 to 150 rpm by using a twin-screw mixer. According to this process, it is difficult to untangle the carbon nanotubes due to the long length L (=100 μm) of carbon nanotubes, as shown in the examples of this China application. Thus, filaments formed from the said carbon nanotubes fail to pass the filter test and possibly break during spinning process. Furthermore, due to the coupling agent and low content of the carbon nanotubes, the increase of the conductivity of the filaments formed according to such a process is limited. In addition, the carbon nanotubes are drawn out during the vacuum dryness procedure for the polyester and carbon nanotubes, and the conductivity of the carbon nanotubes is reduced because of the low content of carbon nanotubes. The masterbatches from the carbon nanotubes prepared by this process exhibit poor physical properties. It is necessary to use bi-component composite spinning method to produce filaments. Moreover, the conductivity of the filaments is lowered and the textiles prepared therefrom merely exhibit an antistatic effect and have a surface resistance of 1.2×106Ω/sq.
Further, CN1584141A discloses conductive composite fibers colored with original liquid by composite spinning process, characterized in that the fibers are composed of a core layer and a sheath layer, wherein the core layer is a polyester containing 2 to 60% of conductive components s elected from a conductive carbon black, a carbon nanotube, a nano-graphite or conductive metal oxides, which has a surface resistance of less than 106Ω·cm. The process of this China application comprises dispersing the conductive particles by melt-state mixing. The masterbatches formed from the carbon nanotube according to this process have unstable physical properties. Thus, it is necessary to use a bi-component spinning procedure to enhance the mechanical properties of fibers.
CN1869291A discloses a fiber structure of nano compound material containing a polyester and a carbon nanotube, wherein the polyester and carbon nanotube are dispersed in a solvent to form a stable dispersion containing polyester/carbon nanotube, and then a fiber structure of nano compound material containing a polyester and a carbon nanotube, such as the structures of a fiber and a non-woven fabric or film formed therefrom is prepared by electrostatic spinning. The formed fiber or non-woven fabric or film has a conductivity of 10−17 to 102 S/cm. In this process, the carbon nanotube is dispersed in the polyester by ultrasonic or mechanical or electromagnetic stirring step.
U.S. Pat. No. 7,094,467B2 discloses an antistatic polymer monofilament and the preparing process thereof. The antistatic polymer monofilament comprises a polymer composite of a thermoplastic polymer as a matrix and carbon nanotubes as a conductive filler. The textiles formed from the monofilament have a surface resistance of 104 to 109 Ω/sq. However, the textiles only exhibit an antistatic effect and haves a surface resistance of 2×107 Ω/sq.
In the conventional technology, carbon nanotubes is generally added in the polymer in an amount of 2 wt. % or less. Such an amount of the carbon nanotubes limits the increase of the conductivity. It is known that the dispersibility of the carbon nanotubes is poor and may result in breakage, brittleness and difficulties in granulating the polymer matrix when the carbon nanotubes in the polyester matrix are in an amount of 5 wt. % or more. The problems and disadvantages stated above may be solved by the addition of a dispersant and a chain extender.
Accordingly, in order to obtain conductive monofilaments containing a low content of carbon nanotubes and having good conductivity and excellent spinning processability as well as to avoid the above-mentioned disadvantages and problems, the present invention provides novel conductive polyester materials, which comprises polyester matrix powders containing a homogeneously dispersed carbon nanotube, a dispersants and a chain extender.